1. Field of the Invention
The present invention relates to inkjet inks, and, more particularly, to a carbon black ink for thermal inkjet printing having improved jetting.
2. Description of the Related Art
Thermal inkjet printing, to which this invention relates, is a commonly practiced form of printing. Thermal ink jet printing involves intense heating of an aqueous ink in a small amount in contact with a heating element so the ink is vaporized. The vaporized ink, including solids in the ink, is then expelled through a nozzle and subsequently jetted to an intended substrate, such as, for example, paper.
The composition of an inkjet ink is traditionally comprised of deionized water, a water-soluble organic solvent, and a colorant. The colorant may be a soluble dye or insoluble pigment. Pigment based inks may have some disadvantages as compared to dye based inks. For example, due to the insolubility of pigments in water, polymeric dispersants and/or surfactants are typically added to improve the dispersibility of the pigment. The addition of a polymeric dispersant can increase the viscosity of an ink, and a viscous ink may be more difficult to jet in a printhead.
It is believed that the properties of the ink composition in a printhead change as water evaporates from the ink adjacent the nozzles of the printhead. Ink formulations having a pigment as a colorant typically also include dispersants and organic components such as humectants, binders, penetrants, organic solvent and the like. As water evaporates from the ink, the percentage of organic components in the ink formulation increases so that the ink becomes less hydrophilic. As the ink becomes less hydrophilic, the dispersed pigment, which is strongly hydrophilic, is pulled back into the bulk aqueous phase, idle time is used to measure this short-term reliability of the ink. “Idle time” means the time between nozzle firings just before the printhead produces delayed or misdirected ink droplets. Typically, pigments, such as carbon black, exhibit poor idle times in inkjet printers.
What is needed in the art is an inkjet ink having a carbon black pigment dispersion that has improved printhead idle time and start up in order to substantially maintain un-interrupted printing with better nozzle jetting.